A TREASURE trove of never-before-seen letters, penned by Jacqueline Kennedy to an Irish priest, shed new light on her belief that US President John F Kennedy would stray in marriage, and the unbearable heartache suffered following his assassination.

A young Jacqueline Bouvier compared her congressman boyfriend to MacBeth, fearing that his all-consuming ambition would eventually drive him into the arms of other women, she wrote to the Reverend Joseph Leonard, a Vincentian priest in Dublin, the Irish Times reported.

‘Glamorous from the outside’ ...Jacqueline and John F Kennedy on holidays with son John Jnr and daughter Caroline. Picture: SuppliedSource:News Limited

“He’s like my father in a way — loves the chase and is bored with the conquest — and once married needs proof he’s still attractive, so flirts with other women and resents you,” Bouvier wrote in July 1952. “I saw how that nearly killed Mummy.”

The famously shy Bouvier hobnobbed with Washington elite in those early years of courtship with JFK, but didn’t always find that lifestyle fulfilling.

“Maybe I’m just dazzled and picture myself in a glittering world of crowned heads and Men of Destiny — and not just a sad little housewife,” she wrote to Leonard in 1953.

“That world can be very glamorous from the outside — but if you’re in it — and you’re lonely — it could be a Hell.”

Bouvier, of course, went on to marry that handsome, up-and-coming politician, who became the nation’s 35th president before falling to an assassin’s bullet after less than two years in office.

The grieving widow told Leonard how she became “bitter against God” and struggled to maintain her deep Catholic faith.

Hopeless ... Jacqueline Kennedy leans over her husband John F Kennedy, moments after he was shot dead during a motorcade through Dallas, Texas. Picture: SuppliedSource:News Limited

“I have to think there is a God — or I have no hope of finding Jack again,” a heartbroken Jackie wrote to Leonard in 1963 following her husband’s murder.

She somehow managed to maintain a sense of humour, even through the worst of times, and comically chastised the Almighty for taking JFK away so early: “God will have a bit of explaining to do to me if I ever see Him.”

Jackie wrote 33 letters to Leonard between 1950 and 1964, before the priest passed away that year. The 130 pages of writings were done on personal stationery, her father-in-law Joseph P Kennedy’s letterhead and official White House stationery.

Bouvier showed incredible fondness for the priest, who she often addressed as “Father L.”

She ended one letter to him: “I REALLY must stop now Father L – but bushels, barrels carts & lorry loads of love to YOU – Jacqueline. XO.”

In another note, on the stationery of Jacqueline Lee Bouvier, she apologised for having not written to him over one summer.

She appreciated how Leonard turned “the other cheek” and wrote her anyway, despite a “long silence” from the future first lady.

Famously shy ... Jacqueline, pictured with husband US President John F Kennedy, hobnobbed with the Washington elite but didn’t always find that lifestyle fulfilling. Picture: AFPSource:AFP

“I have been mentally kicking myself around the block since I got it,” she wrote about receiving a letter from her priest pal.

“You are sweet to say you get pleasure from my letters – but its (sic) nothing compared to the way I adore yours.”

She also wanted to make sure Leonard had her most current address.

On the back on one envelope she mailed to Leonard, Jackie scribbled: “Mrs. John F. Kennedy Hyannisport Mass. USA.”

The writings are up for auction on June 10, through Sheppard’s Irish Auction House in Durrow. The letters are expected to go for between US$550,000 (($587,000) and US$1.1 million.

“We’ve sold exciting pieces before, but this is the most exciting collection ever consigned to an Irish auction house,” director Philip Sheppard told The Post on Tuesday.

“Of course the connection between President Kennedy and Ireland is iconic.”

The auction house declined to reveal the provenance of the letters.

“We’re not commenting [on who owns the letters now],” Mr Sheppard said. “All we’re saying is that they’re from a private source.”

In addition to the 33 letters, the auction house is selling 21 individual pieces of correspondence and pictures associated with Jackie and Leonard.

Those other items — including a Christmas card, signed by both JFK and the first lady, to Leonard, and a condolence note sent to the priest’s rectory following his death — are expected to fetch between US$550 and US$11,000 each.